Definition and controversy

Osteopenia was defined in June 1992 by the World Health Organization. A group of experts decided that condition would mean a bone density that was one standard deviation below that of an average 30-year-old white woman. The group also defined osteoporosis as bone density 2.5 standard deviations or more below that 30-year-old; previously it had been used only in cases where elderly patients had fractured or broken a bone. An osteoporosis epidemiologist at the Mayo Clinic who participated in setting the criteria in 1992 said "It was just meant to indicate the emergence of a problem," and noted that "It didn't have any particular diagnostic or therapeutic significance. It was just meant to show a huge group who looked like they might be at risk."

The definition has been controversial. Steven R. Cummings, of the University of California, San Francisco, said in 2003 that "There is no basis, no biological, social, economic or treatment basis, no basis whatsoever" for using one standard deviation. Cummings added that "As a consequence, though, more than half of the population is told arbitrarily that they have a condition they need to worry about."

Diagnosis

The pharmaceutical company Merck, which sells the anti-bone-loss drug Fosamax, estimated in 2003, from its own market research, that about 8 million women had been found to have osteopenia and about a third of them were taking an osteoporosis drug.

Scans of bones anywhere in the body can be done with X-rays, known as DXA (dual X-ray absorptiometry). Scans can also be done with portable scanners using ultrasound, and portable X-ray machines can measure density in the heel. A study paid for by Merck found that the extent to which osteopenia was diagnosed varied from 28 to 45 percent, depending on the type of machine. Merck was active in promoting deployment of cheaper scanners to be used on extremities, so that they could be used more widely. However, the clinical utility of these scans compared to scans of core portions of the body is disputed.

Causes

Like osteoporosis, osteopenia occurs more frequently in post-menopausal women as a result of the loss of estrogen. It can also be exacerbated by lifestyle factors such as lack of exercise, excess consumption of alcohol, smoking or prolonged use of glucocorticoid medications such as those prescribed for asthma.

The condition can occur in young women who are athletes. It is associated with female athlete triad syndrome as one of the three components, the other two being amenorrhea and disordered eating. Female athletes tend to have lower body weight, lower fat percentage, and higher incidence of asthma than their less active peers. The low estrogen levels (stored in body fat) and/or use of corticosteroids to treat asthma can significantly weaken bone over long periods of time. Distance runners in particular are also discouraged from consuming milk products when training, which would result in lower calcium absorption than other groups.

It is also a sign of normal aging, in contrast to osteoporosis which is present in pathologic aging.

Treatment and controversy

The treatment of osteopenia is controversial. Currently, candidates for therapy include those at the highest risk of osteoporotic bone fracture based on bone mineral density and clinical risk factors. As of 2008, recommendations from the National Osteoporosis Foundation (NOF) are based on risk assessments from the World Health Organization (WHO) Fracture Risk Assessment Tool (FRAX). According to these recommendations, consideration of therapy should be made for postmenopausal women and men older than 50 years of age, if any one of the following is present:

1. Prior hip or vertebral fracture
2. T-score of -2.5 at the femoral neck or spine, excluding secondary causes
3. T-score between -1.0 and -2.5 at the femoral neck or spine and a 10-year probability of hip fracture ≥3% or a 10-year probability of major osteoporotic fracture ≥10%
4. Clinicians' judgment in combination with patient preferences indicate treatment for people with 10-year fracture probabilities above or below these levels.

(Notably, the first two conditions identify individuals with osteoporosis. The third condition corresponds to individuals with osteopenia, namely those with T-scores between -1.0 and -2.5.)

When medical therapy is pursued, treatment includes medications with a range of actions. Commonly used drugs are bisphosphonates including alendronate, risedronate, and ibandronate; selective estrogen receptor modulators (SERMs) such as raloxifene; estrogen; calcitonin; and teriparatide.

Studies have shown that the actual benefits of these drugs may be marginal. Approximately 270 women with osteopenia might need to be treated with drugs for three years so that one of them could avoid a single vertebral fracture.
 

Terms

Results are often reported in 3 terms:

• Measured density in g cm-3
• z-score, the number of standard deviations above or below the mean for the patient's age, sex and ethnicity
• t-score, the number of standard deviations above or below the mean for a healthy 30 year old adult of the same sex and ethnicity as the patient

Limitations

The technique has several limitations.

• Measurement can be affected by the size of the patient, the thickness of tissue overlying the bone, and other factors extraneous to the bones.
• Bone density is a proxy measurement for bone strength, which is the resistance to fracture and the truly significant characteristic. Although the two are usually related, there are some circumstances in which bone density is a poorer indicator of bone strength.
• Reference standards for some populations (e.g., children) are unavailable for many of the methods used.
• Crushed vertebrae can result in falsely high bone density so must be excluded from analysis.

Candidates

The National Osteoporosis Foundation recommends BMD testing for the following individuals:

• All women aged 65 and older regardless of risk factors
• Younger postmenopausal women with one or more risk factors.
• Postmenopausal women who present with fractures (to confirm the diagnosis and determine disease severity).
• Estrogen deficient women at clinical risk for osteoporosis.
• Individuals with vertebral abnormalities.
• Individuals receiving, or planning to receive, long-term glucocorticoid (steroid) therapy.
• Individuals with primary hyperparathyroidism.
• Individuals being monitored to assess the response or efficacy of an approved osteoporosis drug therapy.
• Individuals with a history of eating disorders

Types of tests

While there are many different types of BMD tests, all are non-invasive. Most tests differ in which bones are measured to determine the BMD result.

These tests include:

• Dual energy X-ray absorptiometry (DXA or DEXA)
• Quantitative computed tomography (QCT)
• Qualitative ultrasound (QUS)
• Single photon absorptiometry (SPA)
• Dual photon absorptiometry (DPA)
• Digital X-ray radiogrammetry (DXR)
• Single energy X-ray absorptiometry (SEXA)
• DEXA is currently the most widely used, but ultrasound has been described as a more cost-effective approach to measure bone density.

The test works by measuring a specific bone or bones, usually the spine, hip, and wrist. The density of these bones is then compared with an average index based on age, sex, and size. The resulting comparison is used to determine risk for fractures and the stage of osteoporosis in an individual.

Average bone mineral density = BMC / W [g/cm²]

BMC = bone mineral content = g/cm
W = width at the scanned line

Interpretation

Results are generally scored by two measures, the T-score and the Z-score. Scores indicate the amount one's bone mineral density varies from the mean. Negative scores indicate lower bone density, and positive scores indicate higher.

T-score

The T-score is a comparison of a patient's BMD to that of a healthy thirty-year-old of the same sex and ethnicity. This value is used in post-menopausal women and men over aged 50 because it better predicts risk of future fracture. The criteria of the World Health Organization are:

• Normal is a T-score of -1.0 or higher
• Osteopenia is defined as less than -1.0 and greater than -2.5
• Osteoporosis is defined as -2.5 or lower, meaning a bone density that is two and a half standard deviations below the mean of a thirty year old woman.

Z-score

The Z-score is the number of standard deviations a patient's BMD differs from the average BMD of their age, sex, and ethnicity. This value is used in premenopausal women, men under the age of 50, and in children.